Many different processes are used in the production of modern miniature devices, such as integrated circuits and Micro Electro Mechanical Systems (MEMS). Many such devices are fabricated on a semiconductor wafer substrate. During manufacturing, it is typically most efficient to use techniques that process the entire wafer simultaneously. For example, a layer of metal may be deposited on the entire wafer, a pattern of a photoresist is formed over the metal using photolithography, and then the entire wafer is exposed to an etchant, such as a plasma, that removes the metal where it is not protected by the photoresist.
A plasma is a collection of free charged particles moving in random directions, with the collection being, on average, electrically neutral. Gases fed into plasma generating systems are continuously broken into positive ions and chemically reactive compounds that flow to and react with a work piece surface. The chemically reactive compounds can etch the surface or decompose to deposit material onto the surface. Plasmas can also physically sputter the surface or implant materials into the surface.
Plasmas are used extensively in semiconductor manufacturing to, among other things, remove layers from wafers, to implant materials into wafers, and to deposit materials from wafers. As the size of the substrate on which semiconductors are fabricated increases to 300 mm and larger, the industry has struggled to produce plasma discharges that are uniform over the surface of a wafer so that all areas receive the same amount of processing.
Some processes used in the fabrication of miniature devices are not applied to the whole wafer, but are applied to only a local area. Such methods involve scanning a finely focused ion beam in a pattern over a target surface to mill, etch, or deposit material. Milling involves the direct removal of surface material by the impact of ions in a process called sputtering. In focused ion beam (FIB) deposition, a gas, typically including organometallic compounds, is directed toward the impact point of the FIB on the target surface. The gas decomposes in the presence of the ion beam to add material to the target surface. Ion beam assisted deposition processes are described, for example, in U.S. Pat. No. 5,827,786 to Puretz for “Charged Particle Deposition of Electrically Insulating Films.” FIB-enhanced etching uses a reactive gas in combination with the FIB to increase the number of surface atoms removed by each impinging ion. Such a process is described, for example, in U.S. Pat. No. 5,188,705 to Swanson et al. for “Method of Semiconductor Device Manufacture.”
In FIB deposition and etching, the reactive gas is adsorbed onto the work piece surface and reacts in the presence of the ion beam. The deposition and etch rate is relatively low compared to non-localized processes that simultaneously process the entire wafer. The rate of material removal or deposition depends on the number of charged particles in the beam striking the target surface, the rate at which gas molecules are adsorbed by the surface, and the number of atoms removed or deposited by each charged particle.
While the density of charged particles in a focused ion beam can be relatively high, the beam diameter is typically very small, so the total number of particles striking the surface is relatively small, which means a relatively slow processing rate. Moreover, the high current density in the beam means that gas molecules adhered to the surface area quickly exhausted, and the beam must stop processing the surface until additional gas particles have time to adsorb to the surface.
Because such charged particle beam processes are much slower than whole wafer processing, such processes are typically used for extremely fine work, such as altering prototype circuits, quality control and repair of integrated circuits, preparing sample for a transmission electron microscope, forming probe tips for nanoprofilometers, forming read/write heads for disk drives, and for lithography mask repair.
There are other disadvantages to the use of a focused ion beams for etching and depositing material. Because of the mass and energy of ions in the beam, the beam will inadvertently cause damage to the surface of the substrate and leave implanted ions within the crystal structure of the substrate. This can change the electrical and optical properties of the substrate. This can also change the shape of the features on the substrate, so that subsequent measurements do not accurately characterize the features before processing.
A method of localized processed that has a high deposition or etch rate and that minimizes damage to the substrate is needed.